The present disclosure relates to a power amplifier circuit. Doherty amplifiers are available as high-efficiency power amplifier circuits. A Doherty amplifier typically has a configuration in which a carrier amplifier and a peak amplifier are connected in parallel to each other. The carrier amplifier operates regardless of the power level of an input signal. The peak amplifier is turned off when the power level of the input signal is small and is turned on when the power level of the input signal is large. In this configuration, when the power level of the input signal is large, the carrier amplifier operates while maintaining saturation at a saturation output power level. Accordingly, the Doherty amplifier achieves increased efficiency compared with an ordinary power amplifier circuit.
As a modification of such a Doherty amplifier, for example, U.S. Patent Application Publication No. 2016/0241209 discloses a configuration of detecting a saturation state of a carrier amplifier by using a base current of the carrier amplifier and controlling a bias current of a peak amplifier by using a detection signal. In this configuration, the bias current of the peak amplifier is controlled in accordance with output power, and thereby improved AM-AM characteristics can be obtained compared with a typical Doherty amplifier.
However, in the Doherty amplifier disclosed in U.S. Patent Application Publication No. 2016/0241209, an input signal supplied to the base of the carrier amplifier includes alternating current (AC) components, and thus a detection signal obtained by detecting a base current of the carrier amplifier may include AC components. In this case, the bias circuit of the peak amplifier is controlled by the detection signal including the AC components. Therefore, supply of a bias current to the peak amplifier may become unstable, resulting in degradation of characteristics of output power, such as harmonic distortion.